Copyright | (c) The University of Glasgow, CWI 2001--2004 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | experimental |

Portability | non-portable (local universal quantification) |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

"Scrap your boilerplate" --- Generic programming in Haskell. See http://www.haskell.org/haskellwiki/Research_papers/Generics#Scrap_your_boilerplate.21. This module provides the `Data`

class with its primitives for generic programming, along with instances for many datatypes. It corresponds to a merge between the previous Data.Generics.Basics and almost all of Data.Generics.Instances. The instances that are not present in this module were moved to the `Data.Generics.Instances`

module in the `syb`

package.

For more information, please visit the new SYB wiki: http://www.cs.uu.nl/wiki/bin/view/GenericProgramming/SYB.

module Data.Typeable

class Typeable a => Data a where Source

The `Data`

class comprehends a fundamental primitive `gfoldl`

for folding over constructor applications, say terms. This primitive can be instantiated in several ways to map over the immediate subterms of a term; see the `gmap`

combinators later in this class. Indeed, a generic programmer does not necessarily need to use the ingenious gfoldl primitive but rather the intuitive `gmap`

combinators. The `gfoldl`

primitive is completed by means to query top-level constructors, to turn constructor representations into proper terms, and to list all possible datatype constructors. This completion allows us to serve generic programming scenarios like read, show, equality, term generation.

The combinators `gmapT`

, `gmapQ`

, `gmapM`

, etc are all provided with default definitions in terms of `gfoldl`

, leaving open the opportunity to provide datatype-specific definitions. (The inclusion of the `gmap`

combinators as members of class `Data`

allows the programmer or the compiler to derive specialised, and maybe more efficient code per datatype. *Note*: `gfoldl`

is more higher-order than the `gmap`

combinators. This is subject to ongoing benchmarking experiments. It might turn out that the `gmap`

combinators will be moved out of the class `Data`

.)

Conceptually, the definition of the `gmap`

combinators in terms of the primitive `gfoldl`

requires the identification of the `gfoldl`

function arguments. Technically, we also need to identify the type constructor `c`

for the construction of the result type from the folded term type.

In the definition of `gmapQ`

*x* combinators, we use phantom type constructors for the `c`

in the type of `gfoldl`

because the result type of a query does not involve the (polymorphic) type of the term argument. In the definition of `gmapQl`

we simply use the plain constant type constructor because `gfoldl`

is left-associative anyway and so it is readily suited to fold a left-associative binary operation over the immediate subterms. In the definition of gmapQr, extra effort is needed. We use a higher-order accumulation trick to mediate between left-associative constructor application vs. right-associative binary operation (e.g., `(:)`

). When the query is meant to compute a value of type `r`

, then the result type withing generic folding is `r -> r`

. So the result of folding is a function to which we finally pass the right unit.

With the `-XDeriveDataTypeable`

option, GHC can generate instances of the `Data`

class automatically. For example, given the declaration

data T a b = C1 a b | C2 deriving (Typeable, Data)

GHC will generate an instance that is equivalent to

instance (Data a, Data b) => Data (T a b) where gfoldl k z (C1 a b) = z C1 `k` a `k` b gfoldl k z C2 = z C2 gunfold k z c = case constrIndex c of 1 -> k (k (z C1)) 2 -> z C2 toConstr (C1 _ _) = con_C1 toConstr C2 = con_C2 dataTypeOf _ = ty_T con_C1 = mkConstr ty_T "C1" [] Prefix con_C2 = mkConstr ty_T "C2" [] Prefix ty_T = mkDataType "Module.T" [con_C1, con_C2]

This is suitable for datatypes that are exported transparently.

:: (forall d b. Data d => c (d -> b) -> d -> c b) | defines how nonempty constructor applications are folded. It takes the folded tail of the constructor application and its head, i.e., an immediate subterm, and combines them in some way. |

-> (forall g. g -> c g) | defines how the empty constructor application is folded, like the neutral / start element for list folding. |

-> a | structure to be folded. |

-> c a | result, with a type defined in terms of |

Left-associative fold operation for constructor applications.

The type of `gfoldl`

is a headache, but operationally it is a simple generalisation of a list fold.

The default definition for `gfoldl`

is `const id`

, which is suitable for abstract datatypes with no substructures.

gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a Source

Unfolding constructor applications

toConstr :: a -> Constr Source

Obtaining the constructor from a given datum. For proper terms, this is meant to be the top-level constructor. Primitive datatypes are here viewed as potentially infinite sets of values (i.e., constructors).

dataTypeOf :: a -> DataType Source

The outer type constructor of the type

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c a) Source

Mediate types and unary type constructors. In `Data`

instances of the form `T a`

, `dataCast1`

should be defined as `gcast1`

.

The default definition is `const Nothing`

, which is appropriate for non-unary type constructors.

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c a) Source

Mediate types and binary type constructors. In `Data`

instances of the form `T a b`

, `dataCast2`

should be defined as `gcast2`

.

The default definition is `const Nothing`

, which is appropriate for non-binary type constructors.

gmapT :: (forall b. Data b => b -> b) -> a -> a Source

A generic transformation that maps over the immediate subterms

The default definition instantiates the type constructor `c`

in the type of `gfoldl`

to an identity datatype constructor, using the isomorphism pair as injection and projection.

gmapQl :: forall r r'. (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> a -> r Source

A generic query with a left-associative binary operator

gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> a -> r Source

A generic query with a right-associative binary operator

gmapQ :: (forall d. Data d => d -> u) -> a -> [u] Source

A generic query that processes the immediate subterms and returns a list of results. The list is given in the same order as originally specified in the declaration of the data constructors.

gmapQi :: forall u. Int -> (forall d. Data d => d -> u) -> a -> u Source

A generic query that processes one child by index (zero-based)

gmapM :: forall m. Monad m => (forall d. Data d => d -> m d) -> a -> m a Source

A generic monadic transformation that maps over the immediate subterms

The default definition instantiates the type constructor `c`

in the type of `gfoldl`

to the monad datatype constructor, defining injection and projection using `return`

and `>>=`

.

gmapMp :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a Source

Transformation of at least one immediate subterm does not fail

gmapMo :: forall m. MonadPlus m => (forall d. Data d => d -> m d) -> a -> m a Source

Transformation of one immediate subterm with success

Representation of datatypes. A package of constructor representations with names of type and module.

mkDataType :: String -> [Constr] -> DataType Source

Constructs an algebraic datatype

mkIntType :: String -> DataType Source

Constructs the `Int`

type

mkFloatType :: String -> DataType Source

Constructs the `Float`

type

mkCharType :: String -> DataType Source

Constructs the `Char`

type

mkNoRepType :: String -> DataType Source

Constructs a non-representation for a non-representable type

dataTypeName :: DataType -> String Source

Gets the type constructor including the module

Public representation of datatypes

dataTypeRep :: DataType -> DataRep Source

Gets the public presentation of a datatype

repConstr :: DataType -> ConstrRep -> Constr Source

Look up a constructor by its representation

isAlgType :: DataType -> Bool Source

Test for an algebraic type

dataTypeConstrs :: DataType -> [Constr] Source

Gets the constructors of an algebraic datatype

indexConstr :: DataType -> ConIndex -> Constr Source

Gets the constructor for an index (algebraic datatypes only)

maxConstrIndex :: DataType -> ConIndex Source

Gets the maximum constructor index of an algebraic datatype

isNorepType :: DataType -> Bool Source

Test for a non-representable type

Representation of constructors. Note that equality on constructors with different types may not work -- i.e. the constructors for `False`

and `Nothing`

may compare equal.

Unique index for datatype constructors, counting from 1 in the order they are given in the program text.

Fixity of constructors

mkConstr :: DataType -> String -> [String] -> Fixity -> Constr Source

Constructs a constructor

mkIntegralConstr :: (Integral a, Show a) => DataType -> a -> Constr Source

mkRealConstr :: (Real a, Show a) => DataType -> a -> Constr Source

mkCharConstr :: DataType -> Char -> Constr Source

Makes a constructor for `Char`

.

constrType :: Constr -> DataType Source

Gets the datatype of a constructor

Public representation of constructors

AlgConstr ConIndex | |

IntConstr Integer | |

FloatConstr Rational | |

CharConstr Char |

constrRep :: Constr -> ConstrRep Source

Gets the public presentation of constructors

constrFields :: Constr -> [String] Source

Gets the field labels of a constructor. The list of labels is returned in the same order as they were given in the original constructor declaration.

constrFixity :: Constr -> Fixity Source

Gets the fixity of a constructor

constrIndex :: Constr -> ConIndex Source

Gets the index of a constructor (algebraic datatypes only)

showConstr :: Constr -> String Source

Gets the string for a constructor

readConstr :: DataType -> String -> Maybe Constr Source

Lookup a constructor via a string

tyconUQname :: String -> String Source

Gets the unqualified type constructor: drop *.*.*... before name

tyconModule :: String -> String Source

Gets the module of a type constructor: take *.*.*... before name

fromConstr :: Data a => Constr -> a Source

Build a term skeleton

fromConstrB :: Data a => (forall d. Data d => d) -> Constr -> a Source

Build a term and use a generic function for subterms

fromConstrM :: forall m a. (Monad m, Data a) => (forall d. Data d => m d) -> Constr -> m a Source

Monadic variation on `fromConstrB`

© The University of Glasgow and others

Licensed under a BSD-style license (see top of the page).

https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/base-4.8.2.0/Data-Data.html